and the Knapweeds respond much the same way to cultural and chemical control methods. Our recommendations are just that ... but they have been proven out in the field for years. Once again, it is extremely important that you READ AND FOLLOW THE LABEL with any herbicide you chose to use. In Weber County, the preferred weapon when targeting Diffuse and Spotted Knapweed is Biological Weed Control
(weed feeders). The "time line" featured near the bottom of the page represents a generalized guide to the most effective timing of chemical weed control here in Northern Utah.
Residual herbicides labeled for these species such as Tordon® 22K will do a fine job and keep working when you have packed up and left. Apply at rosette stage when able. Because of it's tenacity, Tordon® will work well on the next years weed crop at any growth stage. 2-4,D ester or 2-4,D amine + Dicamba have good action on these weeds from rosette through medium growth stage. With appropriate adjustments in rates, these herbicides also work very well on pasture or rangelands.
For Bare Ground, we like Arsenal® or Oust® tank mixed with diuron for most all bare ground jobs. For us, it is very cost effective. Your results may vary. See labels for rates on your soils. CAUTION watch sloping ground for potential runoff in the event of heavy rain or overhead irrigation!
Roundup®, of course, can be used on starthistles and knapweeds for general clean-up especially where grasses are mixed in. Keep in mind that no residual chemical action will be left behind, but that can be a good thing where re-planting is planned.
Rights of Way
Knapweeds & Starthistles next to irrigation ditches and streams must be treated with care. There are some amine formulations of 2-4,D that are labeled for aquatic use where grasses under or near the noxious weeds are not desired to be killed. If grasses are of no consequence, we like Rodeo© and the appropriate surfactant.
Along Roadsides, these weeds are just another target for the normal tank mix of the day. 2-4,D & Banvel® (in weed districts of course) are among our favorites. We use Campaign® a lot and see that young rosettes of knapweed and starthistle are taken out. We really get great chemical mowing from the Campaign® also, but with this herbicide, timing is EVERYTHING. The applicator must watch and wait for as many species of target broadleaf weeds to germinate while not letting the roadside grasses get over about 12 inches tall.
Most Labels for herbicides used in crops do not specifically mention Yellow Starthistle or the knapweeds. If rowcrops or alfalfa are being considered for a piece that has been infested with these broadleaf weeds, might be a good idea to plant cereal grains the first year, make the young grain weed free with your favorite chemical, and follow up after harvest when knapweed rosettes again appear with Roundup® or 2-4,D. Then the following year, the tillage and/or chemical treatments you would normally use on rowcrops or alfalfa should keep any remaining invaders in check.
Most appropriate months for treatment of knapweeds and starthistles